The present invention relates to a correction device and correction method for secondary batteries, and more particularly to a correction device and correction method for detecting a variation of terminal voltage of each of 2 or more secondary batteries connected in series and for correcting the variation thereof.
In a nonaqueous solvent secondary battery (e.g., a lithium secondary battery) or a lead secondary battery, a terminal voltage may become too low when it is discharged or left as it is for a long period of time, and too high when it is charged. In this case, the performance of the secondary battery may be deteriorated or the safety may be spoiled. To avoid this, it is necessary to monitor the terminal voltage of these types of secondary battery in order to control the charge and discharge of the secondary battery, so that the terminal voltage is kept within a predetermined range.
In particular, in the case of a lithium secondary battery, when the terminal voltage is 2 V or lower, copper of a collector used in the cathode begins dissolving in an electrolyte and the performance of the second battery is deteriorated. When the terminal voltage is 4.5 V or higher, the electrolyte is discomposed and gas is generated. As a result, the pressure in the secondary battery is increased, so that la safety valve may be operated and the liquid in the battery may be leaked. For this reason, in the case of a lithium secondary battery, charging and discharging operations are generally performed via a protecting circuit which has a function of cutting off a discharge current when the terminal voltage is lowered to a preset discharge prohibiting voltage and rises to a preset charge prohibiting voltage. The discharge prohibiting voltage is set to a value slightly higher than 2 V (e.g., 2.3 V), at which copper of the cathode begins to dissolve. The charge prohibiting voltage is set to a value slightly lower than a value at which the electrolyte begins to decompose (e.g., 4.35 V).
When a protecting circuit is used for a plurality of secondary batteries connected in series, the terminal voltages of the secondary batteries are detected individually and the same protecting operation is performed in the conventional protecting circuit. More specifically, terminal voltages of every secondary batteries are detected, and if the terminal voltage of one of second batteries is lowered to the discharge prohibitihg-voltage, the discharging operation is prohibited, and if the terminal voltage of one of second batteries rises above the charge prohibiting voltage, the charging operation is prohibited. Thus, the secondary batteries are protected.
However, when the conventional protecting circuit as described above is used for a plurality of secondary batteries connected in series, if the amount of charge or self-discharge of a secondary battery differs from that of another, the terminal voltages Of these secondary batteries are also different.
For this reason, during a discharging operation, the terminal voltage of a less-charged secondary battery may reach the discharge prohibiting voltage earlier than the other batteries. In this case, even if the average value of the terminal voltages of the secondary batteries is sufficiently high, the discharge may be stopped. Contrarily, during a charging operation, the terminal voltage of a more-charged secondary battery may reach the charge prohibiting voltage earlier than the other batteries. In this case, the secondary batteries as a whole cannot be fully charged, with the result that they can be used for a shorter period of time.
To avoid the above drawback, a troublesome process is required, i.e., it is necessary to make the terminal voltages equal before the secondary batteries are connected in series.
In an apparatus incorporating a volatile memory device (e.g., RAM), such as a personal computer, when the terminal voltage of a secondary battery used as a power source is lowered, data in the volatile memory device is transferred to a nonvolatile memory device, such as a hard disk, thereby preventing data from being lost. However, if the conventional protecting circuit is used in such an apparatus, the terminal voltage of a secondary battery, which is charged less or self-discharged more as compared to the other batteries, may lower to the discharge prohibiting voltage earlier than the other batteries. In this case, the protecting circuit immediately stops discharging at that time, even if the other secondary batteries have sufficiently high terminal voltages, e.g., even if the average of the terminal voltages of all the secondary batteries is sufficiently high. As a result, data in the volatile memory device is lost.